In a magnetic resonance system or magnetic resonance tomography system, the body to be examined is conventionally exposed with the aid of a basic field magnetic system to a relatively high (e.g., 3 or 7 Tesla) basic field magnetic field (also called a “B0 field”). In addition, a magnetic field gradient is applied with the aid of a gradient system. High-frequency excitation signals (HF signals) are then emitted via a high frequency transmitting system by suitable antenna devices in order to tilt the nuclear spin of certain atoms resonantly excited by this high frequency field so as to be spatially resolved about a defined flip angle with respect to the magnetic field lines of the basic magnetic field. This high frequency excitation or the resulting flip angle distribution will hereinafter also be called core magnetization or “magnetization” for short. With relaxation of the nuclear spin high frequency signals, magnetic resonance signals (also called a “B1 field”) are radiated. The magnetic resonance signals are received by suitable receiving antennae and then processed further. The desired image data can be reconstructed from the raw data thus acquired.
The high frequency signals for nuclear spin magnetization are emitted by a “whole body coil” (also called a “body coil”) or, frequently, by local coils applied to the patient or test person. A typical construction for a whole body coil involves a cage antenna (birdcage antenna), which has a plurality of transmitting rods arranged around a patient area of the tomograph in which a patient is situated during examination so as to run parallel to the longitudinal axis. At the end side, the antenna rods are each annularly capacitively connected to each other.
Previously, whole body antennae have been operated in a “CP mode” (circularly polarized mode). For this purpose, a single temporal HF signal is given to all components of the transmitting antenna (e.g., all transmitting rods of a cage antenna). The pulses having identical amplitude are transferred to the individual components phase-offset and with a shift adapted to the geometry of the transmitting coil. For example, in the case of a cage antenna with 16 rods, the rods are each controlled in a staggered manner using the same HF magnitude signal with 22.5° phase shift. The result is then a high frequency field circularly polarized in the x/y plane (i.e., perpendicular to the longitudinal axis of the cage antenna running in the z direction).
It is possible to individually modify the high frequency signal to be emitted—that is, the incoming sequence of high frequency pulses (referred to herein as a “reference pulse train”)—in terms of amplitude and phase, respectively, by way of a complex transmission scaling factor. As before, it is still possible to operate the antenna in a “CP mode” (i.e., to select the amplitude to be the same for all transmission channels and to merely provide for a phase shift adapted to the geometry of the transmitting coil). Furthermore, depending on the object to be examined, an “EP mode” (elliptically polarized mode) in which the high frequency field is elliptically rather than circularly polarized in the x/y plane is also used. The choice of mode to use typically depends on the shape of the area of the body to be excited. Generally speaking, the CP mode is used with objects that are more cylindrically symmetrical (e.g., regions of the head), whereas the EP mode is used with more elliptical shapes (e.g., regions of the chest or abdomen). The EP mode compensates for non-homogeneities in the B1 field, which are caused by non-circularly symmetrical body shapes.
It is possible to carry out “B1 shimming” of a multi-channel high frequency transmitting system. In this case, the individual transmission scaling factors are calculated on the basis of a patient-specific adjustment with the aim of attaining particularly homogenous excitation compared with the previous standard CP or EP mode.
To calculate the transmission scaling factors, optimizers are used which minimize the magnitude of the difference between the perfectly homogeneously desired target magnetization m and the theoretically attained actual magnetization A·b:b=argbmin(∥A·b−m∥2+β2∥b∥2)  (1).
In equation (1) above, A represents the design matrix comprising a system of linear complex equations, in which, inter alia, the spatial transmitting profiles of the individual transmission channels (e.g., antenna rods) and the existing B0 field distribution are entered. This design matrix is described, for example, by W. Grissom et al. in an article entitled “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation” (Mag. Res. Med., 2006, 56, 620-629). b(t) represents the vector of the HF curves, bC(t)=SFC·bR(t) to be emitted in parallel, wherein SFC is the complex scaling factor for the channel C=1, . . . , N.
If the solution to equation (1)—that is, the minimum of the “target function” defined in equation (1)—is found, the desired scaling factors SF1, SF2, . . . , SFN exist as a result.
Tikhonov regularization (i.e., the second summand in the target function of equation (1)) is used as an expansion of the target function, wherein solutions for small vectors b are preferred which include optimally small high frequency amplitudes. Since the high frequency voltage goes into the calculation of the output power squared, the high frequency exposure (HF exposure) of the patient can thus be reduced with B1 shimming. The HF exposure has to be limited since excessive HF exposure may harm the patient. The HF exposure of the patient is therefore first calculated in advance during planning of the high frequency pulses to be emitted and the high frequency pulses are selected such that a certain limit is not reached. As used herein after, HF exposure refers to a physiological exposure induced by HF radiation rather than the introduced HF energy as such. A typical measure for the HF exposure is what is known as the SAR (Specific Absorption Rate) value, which indicates in units of watts/kg what biological exposure is acting on the patient due to a certain HF pulse power. By way of example, a standardized limit of 4 watts/kg in the “First Level” to the IEC standard applies for the global SAR or HF exposure of a patient. In addition, apart from pre-planning, the SAR exposure of the patient is continuously monitored during examination by way of suitable safety mechanisms on the magnetic resonance system and a measurement is changed or aborted if the HF exposure lies above the provided standards. However, optimally exact planning in advance is expedient to avoid termination of a measurement since this would make a new measurement necessary.
The factor β in equation (1) is a free parameter known as the Tikhonov parameter, which can be selected during solution-finding to choose between optimum homogeneity and optimally low HF exposure.
HF exposure can vary greatly locally since due to B1 shimming, the high frequency pulses are emitted on the individual channels with different amplitude and phase and the overlaying of these pulses (i.e., the mutual cancellation or amplification, which differs from place to place, in the object to be examined) is no longer trivial. There are some areas, therefore, where HF exposure is significantly higher locally than in other regions.
In recent methods, local HF exposure is monitored in the target function in which the HF exposure at particularly defined “virtual observation points” (VOPs) is theoretically calculated. In this regard, local HF exposure does not refer to the HF amplitude which occurs in one place or in a certain volume unit but rather the energy exposure resulting therefrom or the physiological exposure induced by the HF radiation, (e.g., in the form of an Specific Energy Dose or SED value or the SAR value in a certain local volume, such as the VOPs). The HF local exposure value used in the target function can be based, for example, on one or more local SAR values or SED values. This is described for a free individual determination of the high frequency pulse bC(t) by a suitable target function in DE 10 2010 015 044 A1, to which reference can be made hereinafter in relation to the calculation of such VOPs (also called “hot spots”).
If the target function of equation (1) is used for monitoring local HF exposure, neither an approximate qualitative prediction of HF exposure nor the image quality (i.e., the anticipated difference in actual magnetization from the target magnetization) is possible in the Tikhonov regularization when adjusting the parameter β—only whether greater weighting is placed on the image quality or reduced HF exposure during optimization.